A number of thin-film transistors (TFTs) are used as switching elements of a liquid crystal display employing an active matrix driving system and an organic electroluminescence (EL) display.
As the TFT, one in which amorphous silicon or polysilicon is used for a semiconductor layer (channel layer) is known. Recently, in order to improve various characteristics, TFTs in which In (indium)-Zn (zinc)-O (IZO)-based, In—Ga (gallium)-Zn—O (IGZO)-based or Sn (tin)-Zn—O (SZO)-based metal oxide is used for the semiconductor layer are being studied (for example, refer to Patent Literature 1).
Since such thin-film transistors are n-type conductive and exhibit higher channel mobility than those of amorphous silicon and polysilicon, they can be preferably used as switching elements of a high-definition display and a large-screen display. While various theories are being discussed for the mechanism of the n-type conduction, it is mainly said that oxygen vacancy is introduced into the indium oxide structure by oxygen elimination, and as a result, it generates charges to serve as a semiconductor layer. Moreover, since the semiconductor layer having metal oxide as the forming material does not exhibit p-type conduction in principle, and accordingly, has extremely small off-current, use of this thin-film transistor achieves the advantage that power consumption can be reduced.
There is, however, a problem that as to IZO-based, IGZO-based and SZO-based metal oxides which are the metal oxides disclosed in Patent Literature 1, contained Zn, Ga and Sn are liable to react with moisture in the air, as a result, form suboxide which is unstable as the oxide structure for each, and the oxygen vacancy amount cannot be adjusted, which largely deteriorates the transistor characteristics including the drain current, the threshold voltage and the like. Moreover, there is also a problem that while in view of ease of production, silicon oxide (SiO2) is used as the gate insulating film, a thick film thereof is needed to suppress the leakage current between the gates, and as a result, the gate voltage applied to control the electron mobility increases.
To solve these, Patent Literature 2 discloses use of one obtained by adding, to a substance containing at least one element of zinc and tin, at least one of yttrium, niobium, tantalum, hafnium, lanthanum, scandium, vanadium, titanium, magnesium, aluminum, gallium and silicon as metal oxide. Moreover, in order to suppress change in threshold voltage caused by increase of carriers due to the breakdown effect and the radiation effect based on plasma damage in the fabrication stage of the thin-film transistor, it is disclosed that zinc oxide is doped with ions of at least one of gallium, indium, tin, zirconium, hafnium and vanadium (Patent Literature 3). Furthermore, there are reported the electrical characteristics of an oxide film transistor of IZO-based metal oxide doped with tantalum (Non Patent Literature 1). These, however, include the major problem that since zinc and/or tin are included as the main elements in all of the aforementioned cases, the process suffers restriction to a large extent in order to suppress the formation of suboxide in the fabrication stage of the thin-film transistor.
Furthermore, it is also reported that indium oxide doped with any of tin, titanium and tungsten is used as metal oxide instead of IZO or IGZO (Patent Literature 4). As to the oxide film transistor in which indium oxide doped with any of titanium and tungsten disclosed in the aforementioned literature is used as metal oxide, there is, however, the major problem that it is exceedingly difficult to adjust the amount of oxygen vacancy introduced into indium oxide which is the main structure in the fabrication stage of the metal oxide, which loads restriction on the manufacturing process.
Moreover, to solve these, Non Patent Literature 2 reports that an IGZO-based thin-film transistor using yttrium oxide (Y2O3) having the permittivity of 16 which is larger as compared with the permittivity of 3.9 of SiO2 as the gate insulating film is fabricated, and thereby, the thickness of the gate insulating film can be made thin and the gate voltage can be reduced. Moreover, there are reported the transistor characteristics of an IGZO-based thin-film transistor using aluminum oxide (Al2O3) as the gate insulating film (Non Patent Literature 3). Furthermore, it is disclosed to make the gate insulating film of a thin-film transistor be a layered structure of a high permittivity film and a low permittivity film (Patent Literature 5). In all of the aforementioned cases, however, while there is expected the effect that the leakage current can be suppressed with a high permittivity film, i.e., that the physical film thickness can be made thicker as compared with SiO2 by using metal oxide with high permittivity as the gate insulating film in the case where the effective electric field applied to the gate insulating film is the same, there is a problem that since the permittivity of the high permittivity film is not more than 20, the effect is insufficient. Moreover, there is the major problem that control of the threshold voltage which is a noticeable parameter required for the thin-film transistor is not studied at all.
Furthermore, it is reported that in a complementary metal oxide semiconductor (CMOS) device, when using the gate insulating film of layer stack of Al2O3/SiO2 which is a high permittivity film, the flat band voltage shifts due to dipoles at the Al2O3/SiO2 interface (Non Patent Literature 4). Moreover, it is reported that in a layered structure using lanthanum oxide (La2O3) and Y2O3 as high permittivity materials along with the SiO2 layer, dipoles of the opposite direction to that of Al2O3/SiO2 are generated and the flat band voltage shifts to the opposite direction (Non Patent Literature 5). There is, however, the major problem that all of the aforementioned CMOSs have the structure of Si channel/SiO2/the high permittivity film, and that shift of the flat band voltage in the Si channel/the high permittivity film/SiO2 structure is not reported, which loads restriction on the structure.